A.
It was the boarding school where Kaitlin Cooper (Marissa's younger sister)
was sent after Julie married Caleb. It's seen in Episode 24 of Season
3, "The Man of the Year", where Marissa pays a surprise visit
to the school and unexpectedly drops in on Kaitlin.

Kaitlin tells
Marissa that she could be expelled, because the captain of the lacrosse
team lied and told the headmistress that he saw Kaitlin steal a school
crest that was above her door. Kaitlin thinks that he probably has it hidden in
his own locked dorm room, but there's no way to get in there to retrieve
it.

Marissa asks
if Kaitlin can get her one of the school uniforms, and shows up later dressed
provocatively as a very sexy schoolgirl (complete with short skirt & bare midriff). She
tells the lacrosse captain that she is a stripper who has been hired to
entertain him. She takes him to a storage room, gets him
to take off his pants, ties him to a chair, and then (without giving him
the striptease he was expecting), steals his pants (with the keys to his
dorm room in the pocket) and runs for it.

Q.
What is it actually in real life?

A. A former
private mansion, now part of a public park.

Q.
Where can I find it in real life?

A. It is really
the Wattles Mansion, an historic home
& local Hollywood landmark.

You'll find
it on a hill, just above Hollywood Blvd, in Wattles
Park, at 1824 N. Curson Ave.

Built in 1907
as a winter home for Omaha banker Gurdon Wattles, the estate was called
Jualita. It was designed by the same architectural team that built
The Huntington mansion in San Marino and the Beverly
Hills Hotel. The mansion and its gardens became an early Hollywood
tourist attraction.

The opening
view of the school (mansion) was shot looking northwest, towards the front
of the home.

The scenes with
the girls at a school patio table (with brick stairways behind them) were
shot in the formal gardens behind (north of) the Wattles mansion, with the camera looking back south.

Compare The O.C.
photos above to these vintage Hollywood postcards from ages past:

(
You will notice that the postcard above resembles the opening shot
of The O.C. scene at Kaitlin's school. )

(
While the postcard below shows the same area where Marissa, Kaitlin
and her friend her sitting at a table. )

The Wattles
mansion is now run by Hollywood Heritage (a non-profit preservation
organization), and is rented out for weddings, filming and other special
events.

(Scenes from
the Dustin Hoffman/Tom Cruise movie, "Rainman", were also filmed
at Wattles.)

A. I also created
the website Seeing-Stars.com,
a visitor's guidebook to Hollywood, and the Wattles mansion is a Hollywood
landmark. I visited the estate ages ago, when I was first researching the
website. So you would assume I would recognize it when I saw it. A quick look
at the Live Local aerial photos confirmed that it was the same mansion
used for Kirstin's scene.